1. Field of the Invention
The present invention relates to an apparatus for forming an image on a recording-medium according to externally supplied image information.
2. Related Background Art
Electrophotographic printers are capable of image recording with high resolution and high quality. For this reason, there has been developed and widely commercialized various types of printers such as a laser beam printer, LED printer, liquid crystal printer etc. These printers are used for the recording of complex patterns and images, exploiting the feature of high quality image recording.
A controller for generating a complex pattern over a page, for example a postscript controller, requires an image memory for at least a page, such memory being hereinafter called page memory. For example, a page memory of a capacity as large as 1 Mbyte will be required for printing an A4-sized page with a resolution of 300 dpi.
In this manner the output of a high-quality printer contains a considerable amount of information. For this reason the image data handled in the memories of computers or other information processing systems are often encoded or programmed data rather than unprocessed raster image data. Therefore, the performance of a page printer is measured by how rapidly encoded image data of a page can be converted and printed as raster image data of a page.
FIG. 1 schematically shows a laser beam printer (LBP) as a representative example of such a page printer, and FIG. 2 is a block diagram of the control system of such a laser beam printer.
In FIG. 2, external information processing equipment 125 (such as a personal computer or a work station) sends encoded image information (for example in ASCII code, hereinafter referred to as code information) to the laser beam printer through an external interface 127 such as Centronix or RS232C. The code information is received by an interface 118 of the laser beam printer, and then by a microprocessor 117 through an internal bus 128, which has united functions of control bus, address bus and data bus. The microprocessor 117 functions according to a control program stored in a memory 121 composed of a non-volatile ROM.
The microprocessor 117 stores the code information, obtained from the interface 118, after certain processing, in a memory 119 which is a RAM for code information storage. In addition to the storage mentioned above, the microprocessor 117 converts the code information into dot image data and stores the converted data into a memory 120 which is a RAM for image data storage (bit map memory).
A DMA controller 122 reads the data stored in the memory 120 and sends the data to a raster conversion circuit 124. This DMA controller 122 can use the internal bus 128, independently from the microprocessor 117. Upon detecting the image data of a page in the memory 120 (namely completion of conversion of code information of a page into image data), the microprocessor 117 activates the DMA controller 122, which sends the image data from the memory 120 to the raster conversion circuit 124 in response to the request therefrom, occupying the internal bus 128 alternately with the microprocessor 117. The raster conversion circuit 124 converts the parallel image data from the DMA controller 122 into serial image data, which are supplied, in synchronization with a horizontal synchronization signal, to a laser driving circuit in a mechanical control unit 126 for modulating a laser beam.
In addition to the evolution of the code information into the image data, the microprocessor 117 sends various instructions for the printing process to the mechanical control unit of the laser beam printer. An I/O driver 123 functions as an interface between the microprocessor 117 and the mechanical control unit 126.
In the following there will be explained the mechanical control of the laser beam printer, with reference to FIG. 1, in which 1 indicates the main body of the printer. Upon completion of evolution of the code information of a page into image data and their storage in the memory 120, the microprocessor 117 activates a transport motor (not shown). At the same time activated are a photosensitive drum 2, a primary charger 5, a developing roller 7, a transfer charger 34, fixing rollers 15 and discharge rollers 16. The rotation of the above-mentioned motor is controlled by the mechanical control unit 126.
A laser scan unit 3 includes a laser scanning mirror, a laser scanning motor, a laser device, a laser drive circuit etc. The I/O driver 123 activates the laser scan motor in the laser scan unit 3, in addition to the transport motor, and sends high voltages to the primary charger 5, developing roller 7 and transfer charger 34. Furthermore, the I/O driver 123 energizes a clutch (not shown) on a sheet feeding roller, thereby advancing a recording sheet 13 stored in a paper cassette 14. The recording sheet is first stopped at registration rollers 11, and the mechanical control unit 126 informs the I/O driver 123 of the arrival of the recording sheet 13 at the registration rollers 11. When the recording sheet 13 is stopped at the registration rollers 11, the microprocessor 117 activates the DMA controller 122, thereby sending the serial image data from the raster conversion circuit 124 to the laser scan unit 3. The laser beam modulated by the image data irradiates the photosensitive drum to form a latent image thereon, which is then developed into a visible toner image in the developing unit 6.
The recording sheet 13 stopped at the registration rollers 11 is then advanced by these rollers, and the toner image is transferred onto the recording sheet 13 by the transfer charger 34. The sheet 13 bearing the toner is then subjected to thermal fixation by the fixing rollers 15, and is discharged from the apparatus by the discharge rollers 16.
The toner not transferred by the transfer charger 34 is collected by a cleaner 9.
The code information supplied from the external information processing apparatus 135 is thus printed as an image.
Printing of data of plural pages is conducted in according with the timings shown in FIG. 3. At first the microprocessor 117 starts the reception of code information at a timing (a). At the same time the microprocessor 117 starts evolution of the image data and stores the image data in the memory 120. The reception of code information of the 1st page is completed at a timing (b), and that of 2nd page is started at (c). The image data evolution of the 1st page is completed at (d). The transport motor is in rotation in this state, and sheet feeding is conducted at (f). Then the registration rollers 11 are activated at (g), and the DMA controller 122 starts image data reading at (h). Also at the timing (h), the raster conversion circuit 124 generates the serial image data, thus initiating the exposure with the laser beam. At (i), the exposure for the 1st page is completed, and the image data evolution for the 2nd page is started. Thereafter the printing of the 2nd page is conducted with the same sequence as that for the 1st page.
In the chart shown in FIG. 3, the periods from (a) to (d) and from (i) to (j) for image data evolution, and the periods from (h) to (i) for image data reading (also for exposure with the laser beam) are mutually separate and do not mutually overlap, because the capacity of the image memory is limited to one page.
In such process, no access is made to the image memory 120 in the periods from (f) to (h) from (k) to (m), and these is a loss of throughput (number of printed pages per unit time) in the laser beam printer having a long distance from the sheet feed roller 12 to the registration rollers 11. Such loss in the throughput can be avoided by using image memories of two pages and overlapping the period of image data evolution with that of image data reading. However, in such structure, the cost of the image memory will have to be doubled.